Michele Cicoli, Eleonora Di Valentino
In this paper we show how the string landscape can be constrained using observational data. We illustrate this idea by focusing on Fibre Inflation which is a promising class of string inflationary models in type IIB flux compactifications. We determine the values of the microscopic flux-dependent parameters which yield the best fit to the most recent cosmological datasets.
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We study reheating in some one and two field realizations of {\it Fibre Inflation}. We find that reheating begins with a phase of preheating in which long wavelength fluctuation modes are excited. In two field models there is a danger that the parametric amplification of infrared fluctuations in the second scalar field - associated with an entropy mode - might induce an instability of the curvature fluctuations. We show that, at least in the models we consider, the entropy mode has a sufficiently large mass to prevent this instability. Hence, from the point of view of reheating the models we c…
Warming up the Fibres
本研究将IIB型弦通量紧化下的纤维暴胀模型从冷暴胀拓展至温暴胀框架，验证了四种不同修正平衡下的暴胀势均符合观测数据，且温暴胀可拓展模型参数范围、实现亚普朗克场位移。
Fibre inflation is a specific string theory construction based on the Large Volume Scenario that produces an inflationary plateau. We outline its relation to $α$-attractor models for inflation, with the cosmological sector originating from certain string theory corrections leading to $α=2$ and $α=1/2$. Above a certain field range, the steepening effect of higher-order corrections leads first to the breakdown of single-field slow-roll and after that to the onset of 2-field dynamics: the overall volume of the extra dimensions starts to participate in the effective dynamics. Finally, we propose e…
We construct explicit examples of fibre inflation models which are globally embedded in type IIB orientifolds with chiral matter on D7-branes and full closed string moduli stabilisation. The minimal setup involves a Calabi-Yau threefold with h^{1,1}=4 Kaehler moduli which features multiple K3 fibrations and a del Pezzo divisor supporting non-perturbative effects. We perform a consistent choice of orientifold involution, brane setup and gauge fluxes which leads to chiral matter and a moduli-dependent Fayet-Iliopoulos term. After D-term stabilisation, the number of Kaehler moduli is effectively …
Effect of Moduli Redefinitions on Fibre Inflation
本文探讨了通过基模重新定义实现的新纤维膨胀途径，其谱指数与ACT数据一致，产生0.008至0.01范围的张量-标量比，同时结合弦环修正和模重新定义，探索本质场以解释晚期加速。
We present concrete embeddings of fibre inflation models in globally consistent type IIB Calabi-Yau orientifolds with closed string moduli stabilisation. After performing a systematic search through the existing list of toric Calabi-Yau manifolds, we find several examples that reproduce the minimal setup to embed fibre inflation models. This involves Calabi-Yau manifolds with $h^{1,1}= 3$ which are K3 fibrations over a $\mathbb{P}^1$ base with an additional shrinkable rigid divisor. We then provide different consistent choices of the underlying brane set-up which generate a non-perturbative su…
In type IIB Fibre Inflation models the inflaton is a Kaehler modulus which is kinetically coupled to the corresponding axion. In this setup the curvature of the field space induces tachyonic isocurvature perturbations normal to the background inflationary trajectory. However we argue that the associated instability is unphysical since it is due to the use of ill-defined entropy variables. In fact, upon using the correct relative entropy perturbation, we show that in Fibre Inflation axionic isocurvature perturbations decay during inflation and the dynamics is essentially single-field.
Assisted Fibre Inflation in Perturbative LVS
本文提出一种多场纤维膨胀场景，通过多场动态抑制超普朗克尺度位移，展示多纤维模数协同产生与实验一致的宇宙观测结果。
Generic features of models of inflation obtained from string compactifications are the correlations between the model parameters and the postinflationary evolution of the universe. Thus, the postinflationary evolution depends on the inflationary model parameters and accurate inflationary predictions require that this be incorporated in the evolution of the primordial spectrum. The fibre inflation model is a promising model of inflation constructed in type IIB string theory. This model has two interesting features in its postinflationary evolution. The reheating temperature of the model is dire…
We present constraints on the reheating era within the string Fibre Inflation scenario, in terms of the effective equation-of-state parameter of the reheating fluid, $w_{reh}$. The results of the analysis, completely independent on the details of the inflaton physics around the vacuum, illustrate the behavior of the number of $e$-foldings during the reheating stage, $N_{reh}$, and of the final reheating temperature, $T_{reh}$, as functions of the scalar spectral index, $n_s$. We analyze our results with respect to the current bounds given by the PLANCK mission data and to upcoming cosmological…
We study perturbative reheating at the end of fibre inflation where the inflaton is a closed string modulus with a Starobinsky-like potential. We first derive the spectral index $n_s$ and the tensor-to-scalar ratio $r$ as a function of the number of efoldings and the parameter $R$ which controls slow-roll breaking corrections. We then compute the inflaton couplings and decay rates into ultra-light bulk axions and visible sector fields on D7-branes wrapping the inflaton divisor. This leads to a reheating temperature of order $10^{10}$ GeV which requires $52$ efoldings. Ultra-light axions contri…
Chiral global embedding of Fibre Inflation with $\overline{\rm D3}$ uplift
本文研究纤维膨胀的左手全局嵌入与$\overline{\rm D3}$抬升的挑战，提出一个$h^{1,1}=4$的卡拉比-扬流形例子，探讨参数空间约束及可观测的膨胀动力学。
The lack of power on large scales (l < 40) might have been observed by the PLANCK satellite. We argue that this putative feature can be explained by a phase of fast roll at the onset of inflation. We show that in the context of single field models what is required is an asymmetric inflection point model of which fibre inflation is a string motivated example. We study the ability of fibre inflation to generate a suppression of the CMB 2-point function power at low l, finding that the potential derived from string loops is not steep enough for this purpose. We introduce a steeper contribution to…
We perform a detailed analytical and numerical analysis of the multi-field evolution of Fibre Inflation and show that, regardless of the microscopic realisation of the model, the mass-squared of one of the two ultra-light axions becomes always negative during inflation. This implies that the corresponding isocurvature perturbations experience a potential geometrical destabilisation which seems to bring the system away from the perturbative regime. Therefore we conclude that a full understanding of the inflationary evolution of Fibre Inflation can be achieved only via a non-perturbative analysi…
Starobinsky Inflation from String Theory?
本文研究将Starobinsky暴胀嵌入弦论的可行性，通过分析IIB型Kähler模的势能与物质耦合，发现体积模、纤维模和吹胀模均无法同时满足正确势能与共形耦合，表明该嵌入相当困难。
We present a single-field string inflationary model which allows for the generation of primordial black holes in the low mass region where they can account for a significant fraction of the dark matter abundance. The potential is typical of type IIB fibre inflation models and features a plateau at CMB scales and a near inflection point at large distance scales where the power spectrum is enhanced due to a period of ultra slow-roll. The tunability of the underlying parameters is guaranteed by scanning through the string landscape and their stability against quantum corrections is ensured by an …
Spectator Axions in String Inflation and Primordial Black Holes
本研究探究弦理论纤维暴胀中轴子的动力学，发现亚普朗克衰变常数轴子多不影响曲率扰动功率谱，特定参数轴子可增强扰动以辅助原初黑洞成核。
Preheating Axions in String Cosmology
本研究针对弦暴胀模型中的轴子参数共振产生展开分析，通过推导动力学耦合与模依赖轴子质量导致的 Mathieu 方程推广形式，确定共振强度并应用于IIB型纤维暴胀模型，发现重轴子大量产生会约束参数空间以避免宇宙过闭合。
Calculating the power spectrum in stochastic inflation by Monte Carlo simulation and least squares curve fitting
本文提出一种基于蒙特卡洛模拟和最小二乘拟合的新方法，用于计算随机膨胀中的功率谱，通过减少计算成本提高效率。
Fitting NANOGrav 15-year data and ACT data with modified inflation in entropic cosmology
本文通过修正的熵宇宙学膨胀模型，拟合NANOGrav 15年数据和ACT数据，以解释随机引力波背景与观测数据的兼容性。
We consider a class of exponential models of $f(R)$ gravity, in the presence of a canonical scalar field, for which at early times the effective Lagrangian of the theory becomes that of a rescaled canonical scalar field with the Einstein-Hilbert term becoming $\sim αR$, with $α$ a dimensionless constant. This rescaled Einstein-Hilbert scalar field theory at early times alters the inflationary phenomenology of well-known scalar field models of inflation, but more importantly, in the context of this rescaled theory, the Swampland criteria are easily satisfied, assuming that the scalar field is s…
We propose a simple analytic fit for the power spectrum of scalar (curvature) perturbations during inflation, in order to describe slow roll of inflaton and formation of primordial black holes in the early universe, in the framework of single-field models. Our fit is given by a sum of the power spectrum in the slow-roll approximation, needed for a viable description of the cosmic microwave background radiation in agreement with Planck/BICEP/Keck measurements, and the log-normal (Gaussian) fit for the power spectrum enhancement (peak) needed for efficient production of primordial black holes. W…
Higher Derivative Corrections to String Inflation
本文定量研究IIB弦紧致化中N=1超引力的高阶导数修正对模稳定和LVS胀胀模型的影响，通过扫描Kreuzer-Skarke数据库对h^{1,1}≤5的除子拓扑进行分类，并数值验证这些修正不破坏宇宙学观测预言。
We derive the spectrum and analyse the detectability prospects of secondary gravity waves (GWs) associated to primordial black hole (PBH) production in a class of string inflationary models called Fibre Inflation. The inflationary potential features a near inflection point that induces a period of ultra slow-roll responsible for an enhancement of the scalar perturbations which can lead to PBHs with different masses and contributions to dark matter (DM) in agreement with current observational bounds, including CMB constraints on the scalar spectral index and the tensor-to-scalar ratio. This enh…
From Inflation to Quintessence: a History of the Universe in String Theory
本文提出一个IIB型四维弦模型，通过两个稳定模及其超轻轴子，统一描述从暴胀到精质的宇宙演化，并解释暗能量与暗物质的可能来源。
We study a possible origin of the anomalous suppression of the power spectrum at large angular scales in the cosmic microwave background within the framework of explicit string inflationary models where inflation is driven by a closed string modulus parameterizing the size of the extra dimensions. In this class of models the apparent power loss at large scales is caused by the background dynamics which involves a sharp transition from a fast-roll power law phase to a period of Starobinsky-like slow-roll inflation. An interesting feature of this class of string inflationary models is that the n…
The leading order dynamics of the type IIB Large Volume Scenario is characterised by the interplay between $α'$ and non-perturbative effects which fix the overall volume and all local blow-up modes leaving (in general) several flat directions. In this paper we show that, in an arbitrary Calabi-Yau with at least one blow-up mode resolving a point-like singularity, any remaining flat directions can be lifted at subleading order by the inclusions of higher derivative $α'$ corrections. We then focus on simple fibred cases with one remaining flat direction which can behave as an inflaton if its pot…
The influence of cosmological constant type dark energy in the early universe is investigated. This is accommodated by a new dispersion relation in de Sitter spacetime. We perform a global fitting to explore the cosmological parameters space by using the CosmoMC package with the recently released Planck TT and WMAP Polarization datasets. Using the results from global fitting, we compute a new CMB temperature-temperature spectrum. The obtained TT spectrum has lower power compared with the one based on $Λ$CDM model at large scales.
We develop a cosmological model where primordial inflation is driven by a 'solid', defined as a system of three derivatively coupled scalar fields obeying certain symmetries and spontaneously breaking a certain subgroup of these. The symmetry breaking pattern differs drastically from that of standard inflationary models: time translations are unbroken. This prevents our model from fitting into the standard effective field theory description of adiabatic perturbations, with crucial consequences for the dynamics of cosmological perturbations. Most notably, non-gaussianities in the curvature pert…
The latest Planck results show a power deficit in the temperature anisotropies near $\ell \approx 20$ in the cosmic microwave background (CMB). This observation can hardly be explained within the standard inflationary $Λ$-cold-dark-matter ($Λ$CDM) scenario. In this paper we consider a string theory inspired inflationary model (axion monodromy inflation) with a step-like modulation in the potential which gives rise to observable signatures in the primordial perturbations. One interesting phenomenon is that the primordial scalar modes experience a sudden suppression at a critical scale when the …
The observational data on the anisotropy of the cosmic microwave background constraints the scalar spectral tilt $n_s$ and the tensor to scalar ratio $r$ which depend on the first and second derivatives of the inflaton potential. The information can be used to reconstruct the inflaton potential in the polynomial form up to some orders. However, for some classes of potentials, $n_s$ and $r$ behave as $n_s(N)$ and $r(N)$ universally in terms of the number of e-folds $N$. The universal behaviour of $n_s(N)$ can be used to reconstruct a class of inflaton potentials. By parametrizing one of the par…
ACT stands for Awkward Cosmology Theories
研究阿塔卡马宇宙学望远镜（ACT）观测与斯塔罗宾斯基暴胀预测的张力，通过重新审视纳入高曲率算符和修改再加热动力学两种策略，发现拟合ACT数据存在问题，表明使两者相符需难以从紫外完备理论解释的成分，削弱了原始模型简约性。
A second order pole in the scalar kinetic term can lead to a class of inflation models with universal predictions referred to as pole inflation or $α$-attractors. While this kinetic structure is ubiquitous in supergravity effective field theories, realising a consistent UV complete model in e.g. string theory is a non-trivial task. For one, one expects quantum corrections arising in the vicinity of the pole which may spoil the typical attractor dynamics. As a conservative estimate of the range of validity of supergravity models of pole inflation we employ the weak gravity conjecture (WGC). We …
Revisiting the Ultraviolet Tail of the Primordial Gravitational Wave
本文研究原初引力波紫外尾端的对数发散问题，发现其源于暴胀到暴胀后阶段的瞬时转变假设，通过考虑有限转变持续时间首次得到半解析能谱表达式，并给出可用于信号搜索的拟合公式。
In [arXiv:1102.1513] we introduced an inflationary scenario, Non-Abelian Gauge Field Inflation or gauge-flation for short, in which slow-roll inflation is driven by non-Abelian gauge field minimally coupled to gravity. We present a more detailed analysis, both numerical and analytical, of the gauge-flation. By studying the phase diagrams of the theory, we show that getting enough number of e-folds during a slow-roll inflation is fairly robust to the choice of initial gauge field values. In addition, we present a detailed analysis of the cosmic perturbation theory in gauge-flation which has man…
We discuss the inflationary paradigm, how it can be tested, and how various models of inflation fare in the light of data from Planck and BICEP2. We introduce inflation and reheating, and discuss temperature and polarisation anisotropies in the cosmic microwave background radiation due to quantum fluctuations during inflation. Fitting observations of the anisotropies with theoretical realisations obtained by varying various parameters of the curvature power spectrum and cosmological parameters enables one to obtain the allowed ranges of these parameters. We discuss how to relate these paramete…
We study the primordial non-Gaussinity predicted from simple models of inflation with a linear potential and superimposed oscillations. This generic form of the potential is predicted by the axion monodromy inflation model, that has recently been proposed as a possible realization of chaotic inflation in string theory, where the monodromy from wrapped branes extends the range of the closed string axions to beyond the Planck scale. The superimposed oscillations in the potential can lead to new signatures in the CMB spectrum and bispectrum. In particular the bispectrum will have a new distinct s…
A generic feature of string compactifications is the presence of many scalar fields, called moduli. Moduli are usually displaced from their post-inflationary minimum during inflation. Their relaxation to the minimum could lead to the production of oscillons: localised, long-lived, non-linear excitations of the scalar fields. Here we discuss under which conditions oscillons can be produced in string cosmology and illustrate their production and potential phenomenology with two explicit examples: the case of an initially displaced volume modulus in the KKLT scenario and the case of a displaced b…
A generic feature of the known string inflationary models is that the same physics that makes the inflaton lighter than the Hubble scale during inflation often also makes other scalars this light. These scalars can acquire isocurvature fluctuations during inflation, and given that their VEVs determine the mass spectrum and the coupling constants of the effective low-energy field theory, these fluctuations give rise to couplings and masses that are modulated from one Hubble patch to another. These seem just what is required to obtain primordial adiabatic fluctuations through conversion into den…
The PLANCK collaboration has determined values for the spectral parameters of the CMB radiation, namely the spectral index $n_s$, its running $α_s$, the running of the running $β_s$, using a growing body of measurements of CMB anisotropies by the Planck satellite and other missions. These values do not follow the hierarchy of sizes predicted by single field, slow roll inflationary theory, and are thus difficult to fit for such inflation models. In this work we present first a study of 49 single field, slow roll inflationary potentials in which we assess the likelyhood of these models fitting t…
For a robust interpretation of upcoming observations from Planck and LHC experiments it is imperative to understand how the inflationary dynamics of a non-minimally coupled Higgs scalar field may affect the degeneracy of the inflationary observables. We constrain the inflationary observables and the Higgs boson mass during observable inflation by fitting the the Higgs inflationary potential directly to WMAP5+BAO+SN data. We obtain a Higgs mass a value of 143.73+14.97/-6.31 GeV at 95% CL for the central value of top quark mass. We also show that the inflation driven by a non-minimally coupled s…
Spectral distortions from promising single and multifield inflationary models
该研究计算了多种有前景的单场与多场暴胀模型的光谱畸变预测，发现单场模型偏差0.5%-20%，多场混合吸引子信号强约100倍，为检验暴胀理论提供新途径。
We first derive the primordial power spectra, spectral indices and runnings of both scalar and tensor perturbations of a flat inflationary universe to the second-order approximations of the slow-roll parameters, in the framework of loop quantum cosmology with the inverse-volume quantum corrections. This represents an extension of our previous work in which the parameter $σ$ was assumed to be an integer, where $σ$ characterizes the quantum corrections and in general can take any of values from the range $σ\in (0, 6]$. Restricting to the first-order approximations of the slow-roll parameters, we…
Inflationary cosmology with a preceding nonsingular bounce can lead to changes on the primordial density fluctuations. One significant prediction is that the amplitude of the power spectrum may undergo a jump at a critical scale. In this Letter we propose a phenomenological parametrization of the primordial power spectrum in this scenario and confront the jump feature with latest cosmological data. Performing a global fitting, we utilize this possibility to derive a novel method for constraining bounce parameters via cosmological measurements. Combining the CMB, LSS and SNIa data, our result i…
Observational constraints on the second-order primordial power spectrum: Exploring a Continuous Spontaneous Localization inspired inflationary model
本文研究受 CSL 启发的量子坍缩暴胀模型，将原初功率谱计算至哈勃流函数二阶，并用观测数据约束参数以解释标量谱跑动对幂律的偏离。
Combining the latest Planck, Wilkinson Microwave Anisotropy Probe (WMAP), and baryon acoustic oscillation (BAO) data, we exploit the recent cosmic microwave background (CMB) B-mode power spectra data released by the BICEP2 collaboration to constrain the cosmological parameters of the $Λ$CDM model, esp. the primordial power spectra parameters of the scalar and the tensor modes, $n_s,α_s, r, n_t$. We obtained constraints on the parameters for a lensed $Λ$CDM model using the Markov Chain Monte Carlo (MCMC) technique, the marginalized $68\%$ bounds are $r = 0.1043\pm_{0.0914}^{0.0307}$, $n_s = 0.9…
The Dynamics of Reheating in Loop Quantum Cosmology
本文首次在回环量子宇宙学中整合再加热机制，通过比较观测数据验证其与宇宙学约束的一致性，并发现更宽松的e-fold数下界。
The lack of large-angle correlations in the observed microwave background temperature fluctuations persists in the final-year maps from WMAP and the first cosmological data release from Planck. We find a statistically robust and significant result: p-values for the missing correlations lying below 0.24 per cent (i.e. evidence at more than 3 sigma) for foreground cleaned maps, in complete agreement with previous analyses based upon earlier WMAP data. A conservative cut-sky analysis of the Planck HFI 100 GHz frequency band, the `cleanest CMB channel', returns a p-value as small as 0.03 per cent,…
We present constraints on neutrino masses, the primordial fluctuation spectrum from inflation, and other parameters of the $Λ$CDM model, using the one-dimensional Ly$α$-forest power spectrum measured by Palanque-Delabrouille et al. (2013) from SDSS-III/BOSS, complemented by Planck 2015 cosmic microwave background (CMB) data and other cosmological probes. This paper improves on the previous analysis by Palanque-Delabrouille et al. (2015) by using a more powerful set of calibrating hydrodynamical simulations that reduces uncertainties associated with resolution and box size, by adopting a more f…
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